Petroleum is a naturally occurring, complex mixture of hydrocarbons which can exist as solid, liquid or gas depending upon the chemical composition of the petroleum and the temperature and pressure at which the petroleum is confined. Since petroleum is often found in geological formations located at great depths from the earth's surface, it normally contains components which are liquid at the temperatures and pressures existing in the source geological formation, but which components become vapor phase at normal atmospheric conditions existing at the earth's surface.
The most widely used indicator of a crude petroleum's economic value to the producer is its API (American Petroleum Institute) gravity. This indicator is a measure of an oil's density, which is dependent upon its chemical composition, and is also related to specific gravity. Normally, the price which a producer receives for her crude oil depends on its gravity, the less dense oils (higher API gravity) being the most valuable. This price schedule is based on the premise that the lighter oils contain higher percentages of the more desirable products such as gasoline. It is possible that a particular 30.degree. oil may be more valuable than a 20' oil due to a particularly high yield of a desirable product.
As noted above, the surface or "stock tank oil" as finally sold by the producer is not the same liquid which existed in the underground formation from which the petroleum was extracted. The differences between stock tank oil (at the surface) and reservoir oil (underground) are of fundamental importance in understanding the problem and the usefulness of the present invention.
A reservoir oil always contains in solution some components which would become gases at standard temperature and pressure, i.e., 44.7 PSIA and 60.degree. F. The solubility of these gases is due to the elevated pressure and temperature existing at their underground conditions. As the crude oil is produced, brought from the underground reservoir to the earth's surface by either natural or mechanical means, the pressure is decreased until it reaches substantially atmospheric conditions in the stock tanks. This pressure reduction causes certain physical changes in the reservoir fluid properties. Some of the volatile fractions vaporize, causing the liquid volume to shrink, the liquid viscosity to increase, and the API gravity of the crude oil to decrease.
Crude oil, once stored at the surface in atmospheric stock tanks, undergoes a physical change due in part to its chemical composition but more so due to pressure and temperature conditions. When a liquid petroleum is placed in an open container (a container that exposes the liquid to atmospheric conditions), it slowly escapes into the gas phase, eventually leaving the container empty. This is due to the fact that atmospheric pressure is less than the vapor pressures of the major constituents of the crude oil.
Since the physical stat of the crude oil observed when its constituents are vaporizing is in the form of gas, it becomes necessary to apply Boyle's Law; "At constant temperature, the product of the volume and pressure of a given amount of gas is a constant. PxV=constant." In general, the volume of a given quantity of gas at constant temperature is inversely proportional to the pressure. This law when combined with Charles Law and Avogadro's hypothesis can be combined into the "Ideal-Gas Equation", PV=nRT; where P=pressure, V=volume, n=the number of moles of gas, R=Proportionality constant, and T=temperature. The equation expresses a quantitative relationship between all four of the quantities that could possibly be varied in describing the state of gas.
Since the produced crude oil is stored at the surface of the earth at individual, separate well sites having atmospheric stock tanks, it becomes subjected to many changes in relation to the PV=nRT equation. It must be realized that the atmospheric conditions constantly change with respect to any one given geographical location, due to changes in the atmospheric pressures caused by high and low pressure areas that constantly are moving, cold fronts, warm fronts, humidity, solar intensity, the Jet Stream, season of the year, the location of the crude oil storage in relation to the equator, the ground level elevation with respect to sea level, etc. Natural atmospheric conditions are constantly changing and thereby directly effect the vaporization rate of the major constituents of the crude oil. Any change in pressure, volume or temperature with respect to the crude oil at the constantly changing conditions under which it is stored causes a loss of the crude oil to the atmosphere. This loss has been observed to range from 10-30% of the daily produced crude oil volume in barrels (42 U.S. gallons), depending upon the type of crude oil. It has been observed that greater losses occur with higher gravity crude oils.
It should be noted that daily crude oil production varies from well to well, from pool to pool, from field to field and by operator or producer to producer. Most states govern the daily allowance, or maximum amount of oil that can be produced by each individual well, the corresponding producer and the field or pool from which it is produced from underground. The volumes may range from less than 10 barrels of oil per day to many thousands of barrels of oil per day. Present day surface atmospheric oil storage tanks are designed to contain a certain specified volume of crude oil. The volume in each tank is related to the volume which a crude oil purchaser designates as a full load to be hauled away from the well site or tank battery in a mobile truck transport. This volume is typically in the 200 to 220 barrel range. Any additional storage tanks or space available left in a tank is just viewed as additional storage space.
It has been observed that the rate of vaporization due to changes in the atmospheric conditions was so high that, even though a particular well produced commercial quantities of crude oil into the stock tank, none remained to accumulate, or consequently be sold. Since crude oil has a natural vapor pressure that exceeds atmospheric pressure, crude oil is continually lost to the atmosphere. In addition, since PV=nRT, if one changes one of the parameters, the resulting volume changes. Because of the complex chemical nature of crude oils, their constituents and the physical changes they undergo when stored at atmospheric conditions, it becomes virtually impossible to predict the rate of vaporization at any given time or condition. Therefore, to prevent or reduce the volume lost, it becomes necessary to control all of the parameters of the equation PV=nRT within the crude oil storage facilities.
The loss of crude oil to the atmosphere is directly related to the fashion in which it is stored. When liquid petroleum is placed in a closed container, the molecules entering the vapor phase cannot escape to the atmosphere, if the storage container is properly constructed to confine them. In random motion, many molecules strike the liquid and are recaptured. Thus, two processes occur simultaneously: vaporization (entry of particles into the vapor phase) and condensation (entry of particles into the liquid from the vapor phase). The rate of condensation increases as the number of particles in the vapor phase increases. The rate of condensation and the rate of evaporation are compared as a function of time. When the rates of these two processes become equal, the number of particles in the vapor phase becomes stabilized. This steady state, in which there is a balance between the dynamic processes of evaporation and condensation is termed "Dynamic Equilibrium". When equilibrium is reached, there is no further change in the number of particles in the vapor. Vaporization ceases. When the pressure exerted by the vapor is monitored, it is found to increase until equilibrium is reached and thereafter remains constant. It is also observed that the variation in vapor pressure with respect to temperature changes but if the vapor is confined, there is no loss. It is absolutely impossible for crude oil to ever reach dynamic equilibrium with the atmosphere, without loosing the majority of the crude oil to the atmosphere.